Process of making chemical pulp



Aug. 15, 1933. J. oss, JR 1,922,262

PROCESS OF MAKING CHEMICAL PULP Filed Oct. 31. 1930 INVENTOR JAMES.H.ROS 8J5 .AT T ORNEY Patented Aug. 15, 1933 1,922,262

STTS

PROCESS "OF MAKING CHEMECAL PUL? James H. Ross, Jr., Montreal, Quebec, Canada Application October 31, 1930. Serial No. 492,444

2 Claims. (Cl. 92-7) This invention relates to the manufacture of throughout the entire course of the cook. This cellulose or chemical pulp from wood or other naturally requires an excessively high initial confibrous materials More pa t cu a y, the inve centration which, at the temperatures employed tion is chiefly Concerned with t digesting of in commercial practice, is known to have a de- 5 the raw materials and comprises a method and grading action on the cellulose of the fibrous mameans whereby the character or concentration of terlal, This degradation is shown by the visthe active reagents in contact with the fibrous cosity test and the alpha cellulose test. Such materials at any Stage of the digesting D tests indicate a specific characteristic of the fibre may be selectively controlled to maintain condisuch as the micelle size of the molecular agticns necessary for producing pu p of certa n l gregates of the cellulose. For certain purposes it proved characteristics. The aforesaid concentrais essential that the micelle" size be large and ticn 0f the active reagents y, in a da this is particularly true if the pulp produced is to with this invention, be increased decreased at be used in connection with the manufacture of arany desired stage or may be maintained at a tificial ilk.

15 substantially constant predetermined value T ke, for example, the soda process. In thi throughout the Whole y Da i? 0 t d process, the amount of active alkali contained ing process. Likewise, the character of the active i the cooking liquor which is placed in the rereagent in Co tact with the fib us materials al action vessel at the commencement of the cookny' e of op rati n i l j t t l iv ing cycle must be such as to assure a sufficiency control. of the active reagent for pulping throughout the In ordinary mm rci p i h c ipp d entire course of the cook. In ordinary comwood or other raw material to be pulped is charged mercial practice, this requires the use of alkaline into a digester; covered with a solution of the solutions such as caustic soda, having an initial active reagent; and heated until the undesirable concentration of 6 to 7%. The amount of alkali constituents of the material are rendered soluble, in t 1 51 usually ranges from 20 t 25% thus leaving the cellulose pulp as a residue. This of th i ht of t wood contained in the p ss of s ng the raw mat rial is C maction vessel. During the course of the cook n y r ferred t as cook n The hara ter about two thirds to three fourths of this alkali and concentration of the a t, t p racombines with wood constituents so that the final tulle at which the cooking s ca d Out, and concentration is only about one fourth of the $5 the time required to complete the book are subinitial concentration. Hence, in the soda process, J'ect to considerable variation depending, largely, in common with all other chemical processes, the upon the nature of the raw material and the char required initial concentration of the active reacter of pulp to be produced. The correlation agents is necessarily at such a high value as 5 and control of these various factors constitutes to have the aforementioned degrading action on the technique of commercial cellulose or chemical the cellulose and is known to produce a definite D l acture. reduction of the micelle size as shown by low The pulping processes in common use today viscosity and low alphacellulose. There is, theresuch, for example, as the soda, sulphate, keebaf fore, a condition in the carrying out of the soda and sulphite processes do not contemplate the ex process which inhibits the production of a pulp ercise of any selective control with reference to having the micelle size and other characteristhe character or concentration of the active retics best suited for the manufacture of artificial agents in contact with the fibrous material silk and for other purposes for which pulp prothroughout the various stages of the cooking cyduced in accordance with the present invention 16,

cle. I have discovered, however, that such conmay be successfully used.

trol is of prime importance in that it removes cer- The direct advantage of the method of control tain limitations now governing the carrying out afforded by this invention whether embodied in of these processes and enables the production of the soda or other chemical pulping processes now 50 pulp having certain desirable specific characterin use, is that it renders possible a better control istics impossible to obtain under present condiof reaction conditions detrimental to the quality tions from the same-raw materials. of the pulp while maintaining the conditions Inall of the processes referred to, it is necessary necessary for isolation and purification of the at the commencement of the cook to place in fibre. In other words, the character and/or con- 55 the reaction vessel 2. cooking liquor containing an centration of the cooking liquor is sub ect to such active reagent in amounts sufficient to last selective control throughout the cooking cycle as to be at all times of the character or concentration most appropriate for the establishment of the optimum pulping conditions for the stage of cooking then in progress.

The concentration may be controlled in any suitable manner by the periodic or continuous addition of a suitable solution of the cooking liquor at a concentration different from that already contained in the reaction vessel. In practice the particular concentration to be established or maintained for any stage of the cook" will, of course, vary in accordance with the cooking liquor in use; the character of the wood or other fibrous material; the extent to which the cooking has been carried out, and the characteristics desired in the final pulp. The character of the cooking liquor in contact with the fibrous material is subject to selective control in the sense that, at any state of operation, a cooking liquor of characteristics different from that then contained in the reaction vessel may be introduced whenever it is necessary or desirable to promote reaction conditions that would not otherwise occur.

In carrying out the soda process, for example, with the aid of the control afforded by this in vention, it is possible to commence the cook with a cooking liquor having a relatively low initial concentration of about 2% and by successive or continuous addition of a small amount of a suitably concentrated solution of alkali to maintain the initial concentration of 2% of active alkali throughout the cook. Such addition of chemical reagents may be accomplished either manually or automatically by any suitable means. A suggested procedure is the introduction of the said chemicals into an external circulating or heating circuit forming part of the digester installation. The reagents added at any state of operation will usually be of a concentration different from that of the active reagent then in contact with the fibrous materials.

In order to compensate for the added volume of chemical reagents it may, in some instances, be necessary to eliminate an equivalent volume of exhausted or partly exhausted liquor from the reaction vessel. This may be accomplished by providing the reaction vessel with a relief line capable of manual, semi-automatic, or automatic manipulation. This relief line may also be used for extracting portions of the cooking liquor for testing purposes so that the concentration of the chemical reagents in the cooking liquor may be conveniently determined and the addition of fresh reagents signified for introduction into the system to maintain the desired conditions.

In carrying out the soda process in accordance with this invention, it is also advantageous at times to commence the cook with a very weak solution of alkali or with plain water. The wood or other material is heated for a suitable period of time in contact with this starting solution and alkali is then added to raise the concentration to the desired value which is therefore subject to selective variation or may be maintained, constant throughout the remainder of the cook. The concentration employed, after heating with the starting solution may be 2% or any other desired concentration. The end point of the cooking operation may be decided by the amount of alkali actually combined with the wood as measured by tests of the amount and concentration of the cooking liquor.

While the feature of establishing and maintaining a relatively low and substantially constant concentration throughout the whole or major portion of the cook is of important advantage' for certain conditions of operation, this invention also contemplates a much wider range of control. For instance, the concentration of active reagents in contact with the material at one stage of operation may be reduced for a succeeding stage of operation by dilution or by replacing the concentration then in use either in whole or in part by a solution of lower concentration. It is therefore apparent that both the character and the concentration of the cooking liquor in contact with the fibrous material at any given stage is subject to a wide range of control to facilitate the establishment and maintenance of the most favourable pulping conditions throughout the digesting process.

In the foregoing discussion, the invention has been described particularly with reference to the carrying out of the soda process, but it 'will be apparent that similar control and advantages obtain when the invention is embodied in the sulphite or other chemical pulping processes now in use. m

In the further discussion of this invention reference will be had to the single figure of the accompanying drawing wherein I have shown a diagrammatic illustration of a simple form of apparatus suitable for carrying the invention into effect.

Referring more particularly to the drawing, 11 designates a conventional form of digester which is equipped with the usual bottom inlet 12 for the introduction of steam. During the digesting process the cooking liquor is continuously circulated from the bottom to the top of the digester or vice versa through an external circulating and/or heating circuit. This circuit comprises a pump 13 having its inlet connected to the bottom of the digester by a pipe 14 and its outlet connected to the top of the digester by a pipe 15. When external heating is being used a por tion of the pipe 15 passes through a heat interchanger 16 of any suitable construction wherein the circulating liquor is heated prior to being returned to the top of the digester. The pipe 14 is provided with a relief line 16a controlled by a valve 17. By opening the valve 17 a portion of the cooking liquor may be withdrawn from the system for testing purposes or to make room for the liquor-or reagents to be added for the purpose of controlling the concentration of the active reagents in contact with the fibrous material contained in the digester.

The added reagents or cooking liquor may be conveniently introduced into the system inthe following manner. The reserve solution is initially introduced in a supply tank 18 through the medium of a supply line 19 equipped with a valve 20 and a filling funnel 21. From the tank 18 the reserve supply is conducted through a discharge pipe 22 leading to an injector 23 located in the circulating pipe 14 and from thence the reserve supply is drawn into the pump 13 for delivery to the digester. The discharge pipe 22 is controlled by a valve 24 which may be operated in any suitable manner.

Pressure may be supplied to assist in feeding the reserve supply from the tank 18 to the pump 13. For example, a portion of the gas accumulating in the upper portion of the dige y be conducted to the upper porti n o h reserve supply tank 18, through a pipe 25 eq pp with a control valve 26. In this way the pressure exerted on the reserve supply contained in the tank 18 may be adjusted to equalize the pressure of the circulating circuit into which the reserve supply is introduced during the digesting process. The same result may, however, be obtained by the use of steam supplied through a pipe 27 controlled by a valve 28. The pipe 27 is connected to the pipe 25 between the valve 26 and the reserve supply tank 18. As previously explained the character or concentration of the solution or active reagents supplied to the digester from the tank 18 will be predetermined to promote the desired reaction conditions without creating conditions detrimental to the quality of the pulp.

Complete data pertaining to a typical cook conducted in accordance with this invention is given in the appended table, which shows, among other things, the quantity of active reagent (sodium hydroxide) added to the fibrous material, the timing of such additions, and the concentration of active alkali then in contact with the fibrous materials. This table also indicates the yield and other characteristics of the cook conducted as prescribed therein.

Example cook Digester usedspecial design-injector type.

Charge: (A) 3000 grams poplar chips (bone dry weight) (13) 20000 grams water.

Temp. Liqum: Grams NaOH '1 C. injected.

25 0 100 0 509.5 (18 min.)

1. 34 138 279.3 (12 min.) 145 1.95 170 1.65 170 74.1 (0 min.) 172 1.89 170 28.5(l0miu) 170 1. 83 51.3 (5 min.) 170 2.01 170 2. 01 140 (Charge blown from 111 gester) Per cent Yield of screened pulp bone dry 53. 25 Yield of screenings 1. 48

Minutes beating Basis Wt. 25 40 500. 2O 4O 80 100 120 Bursting strength. 53. 5 77. 2 87. 3 95. 3 101.7 111.6 Tearing strength. 62 61 65 74 76 Metres breaking length 5740 (i670 7400 8870 9330 9100 In carrying out this particular cook, the reserve or measuring covering tank 13 is filled with a strong liquor of approximately 20% N 21011 weight. The Wood or other fibrous material is charged into the digester through the top opening 29. A suitable quantity of water is added in the same manner and the opening 29 then sealed by the application of the usual closure (not shown). A heating medium is supplied to the heat interchanger l6 and the pump 13 is set in operation to commence the circulation. The strong solution contained in the tank 18 is fed through the injector 23 at such a rate as to mix evenly with the liquor circulated by the pump 13. The temperature is raised according to the definite time schedule shown on the appended table to a maximum of 170 and the concentration of the active reagent is controlled and predetermined by periodically extracting samples via the relief line 16a and injecting fresh liquor of the necessary concentration. The first sample is usually taken a few minutes after the final addition of the strong liquor supplied from the tank 18. At the conclusion of a predetermined cooking period the fibrous material is blown from the digester and subjected to the usual subsequent treatment such as washing, screening, etc. It will be noted that high liquor ratios were used in the sample cook described but this is not important since the control of concentration afforded by this invention practically eliminates liquor ratio as a factor afiecting the final results.

Having thus described my invention what I claim is:--

1. In the manufacture of chemical pulp the step of maintaining the optimum pulping conditions throughout the course of the digesting process by selective control of the active reagents in contact with the fibrous material, which comprises withdrawing a portion at least of the digesting liquid, adding an active agent thereto in predetermined concentration, and returning the treated portion of the digesting liquid to the digesting process.

2. In the manufacture of chemical pulp by digestion of fibrous material by the action of chemicals in solution, the steps of continuously withdrawing liquid from the digesting solution, adding chemical to said liquid to maintain concentration at a predetermined point, and returning the chemically modified liquid to the digesting process.

JAMES H. ROSS, JR. 

